In recent years, the spread of IC cards has been promoted, and further reduction of thickness thereof is now demanded. Accordingly, it is now required that the thickness of semiconductor chips, which has been about 350 μm, be reduced to 50-100 μm or less. Further, increase of wafer diameter has been studied for improving productivity.
Wafer backgrinding is a conventional step performed after formation of circuit pattern. In the wafer backgrinding, a protective sheet is applied to the circuit surface to protect the circuit surface and to fix the wafer. The protective sheet used in the backgrinding is previously cut into substantially the same shape as the wafer to prevent vibration of the protective sheet during the backgrinding.
The present applicant has made various proposals on the protective sheets; for example, Japanese Patent Applications Nos. 2001-329146 and 2002-67080 propose laminated protective sheets including a rigid film and a stress relaxation film. The use of such laminated protective sheets enables reduction of damage to the wafers because the stress relaxation film reduces the stress occurring during the wafer backgrinding and the rigid film adds strength for wafer transportation.
However, the wafer ground to an ultrathin thickness suffers remarkably lowered strength and is damaged even by a weak impact. For example, a problem occurs as follows. After the backgrinding, the wafers are stored in a wafer cassette and transported to the subsequent steps. The wafers stored in the wafer cassette are usually transported by hand. The transportation often results in chipped wafer edges and cracked wafers by contact of wafer edges to the sidewalls of the wafer cassette.
JP-A-2000-353682 and JP-A-2002-57208 (now JP-A-2003-129011 and JP-A-2003-261842 respectively) disclose techniques wherein a protective sheet is applied to a semiconductor wafer and is cut into a slightly smaller size than the maximum wafer diameter, and the wafer is subjected to subsequent steps such as backgrinding. These techniques can suppress “play” of the protective sheet, so that the vibration of the protective sheet during backgrinding can be reduced. However, they cannot prevent contact of the wafer edges to the sidewalls of the wafer cassette during transportation. Meanwhile, the wafer backgrinding is often followed by application of an adhesive sheet to the ground wafer surface for various purposes such as formation of a die-bonding adhesive layer. After the adhesive sheet is applied, the protective sheet is peeled to transfer the wafer to the adhesive sheet. Herein, the adhesive sheet applied to the wafer is cut in substantially the same diameter as the wafer. The cutting of the adhesive sheet is performed by running a cutter along the outer periphery of the wafer. Accordingly, the adhesive sheet can be cut in substantially the same diameter as the wafer. However, the cutting blade is brought into contact with the outer periphery of the wafer and often damages the wafer.